alkali metals image

Higher and foundation tiers

The alkali metals

alkali metal reactivity list When we think of metals we usually think of a shiny, hard material which is strong and fairly unreactive. However not all metals fit this description, some metals are soft and very reactive. Group 1 in the periodic table is called the alkali metals; it contains the metals lithium, sodium, potassium, rubidium, caesium and francium. These metals are all very reactive and must be handled with great care. Lithium at the top of group 1 is the least reactive and as we go down the group the metals react more and more violently. Since they are in group 1 of the periodic table they all have one electron in their last shell, so losing this electron will give them a full outer electron shell (stable electronic structure). Losing their outer electron will mean they form ions with a +1 charge e.g. Li+, Na+, K+, Rb+, Cs+.

The Properties of alkali metals

The alkali metals are stored in jars filled with oil or paraffin to keep them away from contact with water or the oxygen in the air. They are soft and can all be cut with a knife, lithium is the most difficult to cut but the metals become softer as you go down the group. When freshly cut the metal is silvery and shiny but if left exposed to air the freshly cut surface will react with oxygen and moisture in the air and quickly tarnish.

lithium, sodium and potassium metals in jars.

Physical properties of the alkali metals

The physical properties of the alkali metals are also very different from what you might expect of a typical metal. The table below lists the melting and boiling points of the alkali metals as well as their densities.

alkali metal melting point/0C boiling point/0C density(g/cm3) at 250C
lithium 180 1347 0.53
sodium 98 883 0.97
potassium 63.5 774 0.86
rubidium 39 696 1.53
caesium 28 669 1.87

To compare alkali metals with some other everyday metals compare the values in the table below with the table above.

metal melting point/0C boiling point/0C density(g/cm3) at 250C
aluminium 660 2519 2.7
tungsten 3422 5555 19.3
iron 1535 2861 7.8
titanium 1668 3287 4.5

Identifying trends

By studying the information in the table it is fairly clear that the alkali metals have unusually low melting and boiling points when compared to other metals, their densities are also low when compared to other metals. The density of water is 1 g/cm3, so any metal with a density less than this will float in water. However the alkali metals react violently with water, this is one of the reasons why they are stored in jars containing oil or paraffin, this ensures that water is kept well away from these reactive metals.

The information in the table for the alkali metals shows other trends or patterns:

The alkali metals are all group 1 metals so they will have 1 electron 1 their outer electron energy level. Since the atoms get larger as you descend the group, this outer electron is further away from the attraction of the positive nucleus and so is more easily lost or removed, this means that less energy will be needed to remove it. This means that the reactivity of the alkali metals will increase as you descend the group.
Having just one electron in their outer shell means that when they react; the alkali metals will lose this electron and form ions with a 1+ charge.

Alkali metals- reaction with water

The alkali metals react very violently with water to form an alkaline solution and the flammable gas hydrogen is also released. The image below shows the typical reactions of lithium, sodium and potassium with water. Enough heat is generated in these reactions with the water that the metal may actually melt and form a ball of molten liquid metal that shoots across the surface of the water.

The reaction of water with potassium is violent enough that the hydrogen gas will ignite by itself and burn with a mauve flame (lilac or pale purple). Sodium and lithium react less violently and the hydrogen produced here will not ignite and burn by itself but will have to be lit by a burning splint, however once lit the gas will continue to burn on its own. Sodium colours the hydrogen flame yellow and lithium will turn the burning hydrogen gas brick red.

The image below illustrates the colours of the burning hydrogen flame above the three metals lithium, sodium and potassium. The alkali metals all react violently with water to form alkaline solutions (solutions of metal hydroxides). The reaction of potassium with water is so violent that the hydrogen gas released spontaneously catches fire to give a lilac coloured flame above the metal. Sodium and lithium also release hydrogen but this needs to be lit with a burning splint. The burning hydrogen above sodium is yellow and red with lithium.

alkali metals reacting with water.

If a few drops of universal indicator are added to the water in the glass trough above then it will quickly turn purple once the alkali metals start reacting, showing that a strong alkali has been formed. Rubidium and caesium being at the bottom of group 1 are even more reactive, they are denser than water and will sink but they react in a similar but much more violent way. All the alkali metals react with water in a similar way, they all form solutions of metal hydroxides (alkalis) and release the explosive gas hydrogen, this is outlined in the equations below:

alkali metal(s)   +   water(l)   →   metal hydroxide(aq)   +   hydrogen(g)
e.g. lithium
lithium(s)   +   water(l)   →   lithium hydroxide(aq)   +   hydrogen(g)
2Li(s)   +  2H2O(l)   →   2LiOH(aq)   +   H2(g)
e.g. sodium
sodium(s)   +   water(l)   →   sodium hydroxide(aq)   +   hydrogen(g)
2Na(s)   +  2H2O(l)   →   2NaOH(aq)   +   H2(g)
e.g. potassium
potassium(s)   +   water(l)   →   potassium hydroxide(aq)   +   hydrogen(g)
2K(s)   +  2H2O(l)   →   2KOH(aq)   +   H2(g)

Alkali metals get their name because they react with water to form alkaline solutions, lithium, sodium and potassium hydroxides are all strong alkaline solutions.

Reactions of alkali metals with oxygen and the halogens

Similar violent reactions occur between the alkali metals and oxygen and chlorine gases to form oxides, peroxides, superoxides and chlorides, the same trends are always seen, the reactions become more violent the lower the metal is in group 1. The reason for this is that all alkali metals have 1 electron in their outer (last electron shell), therefore if they can lose this one electron they will end up with a stable or full last shell and as mentioned above less energy is required to remove this outer shell electron the further down group 1 the metal is found. Since the metals are losing 1 electron, the atom will end up with 1 more positively charged proton in the nucleus than negatively charged electrons in its shells, so it will end up forming a positive ion with a charge of +1.

The image below shows a possible set-up for the reaction the alkali metal sodium with chlorine gas. A small piece of sodium is placed ON a mound of sand on the base of the flask. The flask is filled with dry chlorine gas. To start the reaction a drop of water is dropped onto the sodium metal from the pipette. A very violent reaction occurs between the sodium and chlorine; a bright flash is seen as the sodium and chlorine react. The flask fills up with white "smoke"; which is solid sodium chloride.

Sodium and chlorine reacting violenetly in a flat-bottomed flask

Reaction with halogens

The alkali metals react with the halogens (F, Cl, Br, I) to form colourless ionic solids. The reactions are very exothermic and can be violent. The reactions follow the expected trends, the more reactive the alkali metals and the more reactive the halogen the more violent and explosive the reaction. The reaction can be summarised as:

2M(s) + X2(g) 2MX(s)

Where M= any of the alkali metals and X= F, Cl, Br, I

So for example when sodium reacts with chlorine to form sodium chloride we have:

sodium(s)   +   chlorine(l)   →   sodium chloride(s)  
2Na(s)   +  Cl2(g)   →   2NaCl(s)

Reaction with oxygen

The products of the reaction of an alkali metal with oxygen depend on its reactivity. Normally when oxygen reacts it gains 2 electrons to form the oxide ion, O2-. This ion is particularly stable since the oxide ion has full octet or 8 electrons in its outer shell. However in the reactions of oxygen with the alkali metals other ions of oxygen are also formed. One of these ions, the peroxide ion has the formula O2 2- and it forms when sodium and potassium react with oxygen. Reactive alkali metals such as potassium, rubidium and caesium are also able to form an oxide called a superoxide. The superoxide ion has the formula 02-


If a piece of lithium is placed on a burning spoon and heated strongly in air it burns with a brick red flame to form lithium oxide:
lithium + oxygen lithium oxide
4Li + 02 2Li2O


If a piece of sodium is placed on a burning spoon and heated strongly in air it burns with a yellow flame to form a mixture of two products, sodium oxide and sodium peroxide:
sodium + oxygen sodium oxide
4Na + 02 2Na2O
sodium + oxygen sodium peroxide
2Na + 02 Na2O2


Potassium is the most reactive alkali metal studied in GCSE chemistry. The reactive alkali metals are able to form an oxide called a superoxide. The formula for the superoxide ion is O2-.
So when a piece of potassium metal is burned in air a lilac coloured flame is seen and a mixture of two oxides are produced; these are potassium peroxide and potassium superoxide.
potassium + oxygen potassium peroxide
2K + 02 K2O2

and for potassium superoxide we have:

potassium + oxygen potassium superoxide
K + 02 KO2

Key Points

Practice questions

Check your understanding - Questions on the alkali metals

Check your understanding - Additional questions on the alkali metals

Check your understanding - Quick quiz on alkali metals